Owls: formidable predators of the Kenai Peninsula

Refuge Notebook

Posted: Friday, February 16, 2007

By Toby Burke

Late winter is the time of the year when residents of the Kenai Peninsula often become aware of our many owls. Owls are now starting their annual breeding cycle by vocalizing to attract mates and establish breeding territories.

There are eight species of owls that frequent the Kenai Peninsula either as winter visitors, summer breeders, or year-round residents. They are the great gray owl, great horned owl, snowy owl, short-eared owl, northern hawk owl, boreal owl, northern saw-whet owl, and western screech-owl. While most species of owls are nocturnal and a few are diurnal (daytime), almost all of our owl species exhibit increased activity during the early morning or evening twilight periods.

Owls belong to the order Strigiformes, which has about 225 species worldwide. Traditionally, ornithologists considered owls, as nocturnal birds of prey, to be the closest relatives to the order Falconiformes, which is composed of diurnal birds of prey such as eagles, hawks, and falcons. But new taxonomic studies based on DNA indicate that owls may be more closely related to the order Caprimulgiformes, which is composed of whippoorwills, nighthawks, and their allies.

The world’s largest owls  Blakiston’s fish owl, the Eurasian eagle owl, and Verreaux’s eagle owl  are all Old World species. The females of each species may approach weights of 10 pounds, wingspans of nearly 6 feet and body lengths of nearly 30 inches. On the other end of the spectrum, the world smallest owls are the least pygmy owl of South America and the elf owl, which inhabits Mexico and neighboring U.S. border states. The elf owl typically measures 4.8 to 5.5 inches in length and weighs 1.3 to 1.9 ounces. It’s equivalent in size and weight to a large sparrow.

Having evolved as predators in low light conditions, owls have several physical adaptations that make them fearsome hunters. Foremost, they have exceptional vision and hearing, they can fly silently, and they have powerful talons.

Owls have large-forward facing eyes which give them stereoscopic, or three-dimensional, vision like humans. Unlike humans though, their eyes are relatively large, accounting for an incredible one to five percent of their total body mass, depending on the species. Their proportionately large eyes improve their sight under low light conditions by enabling them to collect more light. The eye itself contains an abundance of “rod” cells that aid them in processing the light. These cells are very sensitive to light and movement. Cells that are very sensitive to color are known as “cone” cells. Owls possess few cone cells and these cells are not very sensitive in low light conditions, so most owls see in limited color or in monochrome.

Furthermore, the exceptional light gathering ability is enhanced by a reflective layer behind the eye called the “tapetum lucidum.” This layer reflects back onto the rod cells any light that may have passed through without hitting the rods the first time. The Old World tawny owl is generally acknowledged as having the most well-developed night vision, not only among owls but probably all vertebrates, and it is believed that their night vision is approximately 100 times more sensitive than human vision.

Owl eyes are not really not so much eye “balls” as they are elongated tubes, held in place by a bony structure called the sclerotic ring. Accordingly, owl eyeballs cannot be rolled or moved as humans move their eyes. An owl must rotate, raise, or lower its entire head in order to move its eyes. This is compensated by 14 cervical vertebrae, twice as many as humans, which allow the owl to rotate its head 270 degrees from side-to-side and turn its head straight up if desired.

Owl ears are located behind the eyes. They are covered by feathers of the facial disk which direct sound waves toward the ears. In strictly nocturnal species the ear openings are set somewhat asymmetrically to enhance their ability to triangulate the specific location or direction of a sound. If one were to look at the bare skull of one of these nocturnal owls, the skull would appear slightly lopsided. These owls also have a more pronounced facial disk that can be adjusted to efficiently direct sound waves into the ear. The “ear tufts” found on “eared owls” are not ears at all but simply feathers used for display.

The pinpoint accuracy of an attacking owl on its invisible prey is based on its ability to discern left ear-right ear differences of about 30 millionths of a second. Owls use their remarkable auditory system to detect movement or vocalizations of prey under organic debris, foliage or snow. The owl range of hearing is similar to that of a human but it is more acute at detecting certain frequencies, especially those of its prey species.

Owl plumage has unique flutings, or fimbriae, on the leading edge of primary feathers of the wing. These comb-like structures reduce turbulence that normally forms over the wing surface and creates the distinctive rushing or swooshing sound of flight. The owl’s fimbriae dampen this flight noise so that the owl can fly in silence, capturing prey by stealth and using its hearing to locate prey as it flies. Several strictly daytime owl species have lost this adaptation.

An owl’s foot has four toes and a unique flexible joint. When an owl is perched or grasping prey, two toes face forward and two face rearward. When flying, the outer rear toe on each side swivels around to face the front so that three toes are facing forward and one rearward. The talons are spread wide when the owl is attacking to increase the likelihood of contacting its prey. The bones of its feet are much stronger than most birds in order to withstand the force of impact as it strikes and initially stuns its prey. The underside of the foot also has a coarse, nubby surface that helps it grip its prey and perch. Like most raptors, owls have talons with a mechanism that locks and ratchets down on their prey or perch to avoid the fatigue of continuous muscular contraction.

Keep all these remarkable adaptations in mind the next time you hear an owl calling through the dark of night and be aware that a formidable winged predator is alert and on the prowl.

Toby Burke is a biological technician at the Kenai National Wildlife Refuge. You can report or listen to interesting local bird sightings on the Kenai National Wildlife Refuge Birding Hotline (907) 262-2300.